[PATCH 06/10] xfs: mount api - add xfs_reconfigure()

[Ian Kent <raven@themaw.net>]

Add the fs_context_operations method .reconfigure that performs
remount validation as previously done by the super_operations
.remount_fs method.

An attempt has also been made to update the comment about options
handling problems with mount(8) to reflect the current situation.

Signed-off-by: Ian Kent <raven@themaw.net>
---
 fs/xfs/xfs_super.c |  171 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 171 insertions(+)

diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 0ec0142b94e1..7326b21b32d1 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -1721,6 +1721,176 @@ xfs_validate_params(
 	return 0;
 }
 
+STATIC int
+xfs_reconfigure(
+	struct fs_context *fc)
+{
+	struct xfs_fs_context	*ctx = fc->fs_private;
+	struct xfs_mount	*mp = XFS_M(fc->root->d_sb);
+	struct xfs_mount        *new_mp = fc->s_fs_info;
+	xfs_sb_t		*sbp = &mp->m_sb;
+	int			flags = fc->sb_flags;
+	int			error;
+
+	error = xfs_validate_params(new_mp, ctx, fc->purpose);
+	if (error)
+		return error;
+
+	/*
+	 * There have been problems in the past with options
+	 * passed from mount(8).
+	 *
+	 * The problem being that options passed by mount(8) in
+	 * the case where only the the mount point path is given
+	 * would consist of the existing fstab options with the
+	 * options from mtab for the current mount merged in and
+	 * the options given on the command line last. But the
+	 * result couldn't be relied upon to accurately reflect the
+	 * current mount options so that rejecting options that
+	 * can't be changed on reconfigure could erronously cause
+	 * mount failure.
+	 *
+	 * The mount-api uses a legacy mount options handler
+	 * in the VFS to accomodate mount(8) so these options
+	 * will continue to be passed. Even if mount(8) is
+	 * updated to use fsopen()/fsconfig()/fsmount() it's
+	 * likely to continue to set the existing options so
+	 * options problems with reconfigure could continue.
+	 *
+	 * For the longest time mtab locking was a problem and
+	 * this could have been one possible cause. It's also
+	 * possible there could have been options order problems.
+	 *
+	 * That has changed now as mtab is a link to the proc
+	 * file system mount table so mtab options should be
+	 * always accurate.
+	 *
+	 * Consulting the util-linux maintainer (Karel Zak) he
+	 * is confident that, in this case, the options passed
+	 * by mount(8) will be those of the current mount and
+	 * the options order should be a correct merge of fstab
+	 * and mtab options, and new options given on the command
+	 * line.
+	 *
+	 * So, in theory, it should be possible to compare incoming
+	 * options and return an error for options that differ from
+	 * the current mount and can't be changed on reconfigure to
+	 * prevent users from believing they might have changed mount
+	 * options using remount which can't be changed.
+	 *
+	 * But for now continue to return success for every reconfigure
+	 * request, and silently ignore all options that can't actually
+	 * be changed.
+	 */
+
+	/* inode32 -> inode64 */
+	if ((mp->m_flags & XFS_MOUNT_SMALL_INUMS) &&
+	    !(new_mp->m_flags & XFS_MOUNT_SMALL_INUMS)) {
+		mp->m_flags &= ~XFS_MOUNT_SMALL_INUMS;
+		mp->m_maxagi = xfs_set_inode_alloc(mp, sbp->sb_agcount);
+	}
+
+	/* inode64 -> inode32 */
+	if (!(mp->m_flags & XFS_MOUNT_SMALL_INUMS) &&
+	    (new_mp->m_flags & XFS_MOUNT_SMALL_INUMS)) {
+		mp->m_flags |= XFS_MOUNT_SMALL_INUMS;
+		mp->m_maxagi = xfs_set_inode_alloc(mp, sbp->sb_agcount);
+	}
+
+	/* ro -> rw */
+	if ((mp->m_flags & XFS_MOUNT_RDONLY) && !(flags & SB_RDONLY)) {
+		if (mp->m_flags & XFS_MOUNT_NORECOVERY) {
+			xfs_warn(mp,
+		"ro->rw transition prohibited on norecovery mount");
+			return -EINVAL;
+		}
+
+		if (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5 &&
+		    xfs_sb_has_ro_compat_feature(sbp,
+					XFS_SB_FEAT_RO_COMPAT_UNKNOWN)) {
+			xfs_warn(mp,
+"ro->rw transition prohibited on unknown (0x%x) ro-compat filesystem",
+				(sbp->sb_features_ro_compat &
+					XFS_SB_FEAT_RO_COMPAT_UNKNOWN));
+			return -EINVAL;
+		}
+
+		mp->m_flags &= ~XFS_MOUNT_RDONLY;
+
+		/*
+		 * If this is the first remount to writeable state we
+		 * might have some superblock changes to update.
+		 */
+		if (mp->m_update_sb) {
+			error = xfs_sync_sb(mp, false);
+			if (error) {
+				xfs_warn(mp, "failed to write sb changes");
+				return error;
+			}
+			mp->m_update_sb = false;
+		}
+
+		/*
+		 * Fill out the reserve pool if it is empty. Use the stashed
+		 * value if it is non-zero, otherwise go with the default.
+		 */
+		xfs_restore_resvblks(mp);
+		xfs_log_work_queue(mp);
+
+		/* Recover any CoW blocks that never got remapped. */
+		error = xfs_reflink_recover_cow(mp);
+		if (error) {
+			xfs_err(mp,
+	"Error %d recovering leftover CoW allocations.", error);
+			xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_INCORE);
+			return error;
+		}
+		xfs_start_block_reaping(mp);
+
+		/* Create the per-AG metadata reservation pool .*/
+		error = xfs_fs_reserve_ag_blocks(mp);
+		if (error && error != -ENOSPC)
+			return error;
+	}
+
+	/* rw -> ro */
+	if (!(mp->m_flags & XFS_MOUNT_RDONLY) && (flags & SB_RDONLY)) {
+		/*
+		 * Cancel background eofb scanning so it cannot race with the
+		 * final log force+buftarg wait and deadlock the remount.
+		 */
+		xfs_stop_block_reaping(mp);
+
+		/* Get rid of any leftover CoW reservations... */
+		error = xfs_icache_free_cowblocks(mp, NULL);
+		if (error) {
+			xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_INCORE);
+			return error;
+		}
+
+		/* Free the per-AG metadata reservation pool. */
+		error = xfs_fs_unreserve_ag_blocks(mp);
+		if (error) {
+			xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_INCORE);
+			return error;
+		}
+
+		/*
+		 * Before we sync the metadata, we need to free up the reserve
+		 * block pool so that the used block count in the superblock on
+		 * disk is correct at the end of the remount. Stash the current
+		 * reserve pool size so that if we get remounted rw, we can
+		 * return it to the same size.
+		 */
+		xfs_save_resvblks(mp);
+
+		xfs_quiesce_attr(mp);
+		mp->m_flags |= XFS_MOUNT_RDONLY;
+	}
+
+	return 0;
+}
+
 /*
  * Second stage of a freeze. The data is already frozen so we only
  * need to take care of the metadata. Once that's done sync the superblock
@@ -2246,6 +2416,7 @@ static const struct super_operations xfs_super_operations = {
 static const struct fs_context_operations xfs_context_ops = {
 	.parse_param = xfs_parse_param,
 	.get_tree    = xfs_get_tree,
+	.reconfigure = xfs_reconfigure,
 };
 
 static struct file_system_type xfs_fs_type = {